The disclosure of Japanese Patent Application No. 2000-033660 filed on Feb. 10, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a device and a method for detecting the speed of a braking operation, and to a device and a method for controlling braking forces applied to rear wheels with respect to braking forces applied to front wheels.
2. Description of Related Art
If a brake pedal is depressed quickly during a hard braking operation, the deceleration of a vehicle body increases substantially, and a substantial amount of load is displaced to the front part of the vehicle body. As a result, the load applied to front wheels increases, and the load applied to rear wheels decreases. If an equal brake fluid pressure is transmitted to brake devices for the front and rear wheels under such circumstances, the rear wheels tend to become locked. If the rear wheels are locked, the attitude of the vehicle may be affected. Thus, there is known an electronic braking force distribution (EBD) system that electrically controls, if such hard braking has been detected, the distribution of the brake fluid pressures for the rear wheels with respect to the brake fluid pressures for the front wheels such that the braking forces generated in the brake devices for the rear wheels are reduced with respect to the braking forces generated in the brake devices for the front wheels (as disclosed, e.g., in Japanese Patent Application Laid-Open No. 5-147520).
In Japanese Patent Application Laid-Open No. 5-147520, the brake fluid pressure force-fed from a master cylinder is detected by a pressure sensor, and it is judged from the state of changes in the detected brake fluid pressure whether or not a hard braking operation has been performed. Accordingly, this system requires the provision of a separate pressure sensor for detecting a hard braking operation. Further, if there arises a malfunction in the detection system including the pressure sensor, it is impossible to judge whether or not a hard braking operation has actually been performed.
The invention has been made to address such problems. It is one object of the invention to provide a braking operation speed detecting device capable of detecting the brake operating speed from quantities indicative of the state of a vehicle detected by existing sensors. It is also an object of the invention to provide a vehicular brake control device for controlling braking forces applied to rear wheels with respect to braking forces applied to front wheels using the detection result of the brake operating speed detecting device.
In order to address the aforementioned problems, a brake operating speed detecting device according to a first aspect of the invention is designed to: (a) filter vehicle body decelerations sequentially obtained based on rotational speeds of wheels by means of a plurality of filters having different cut-off frequencies, and (b) detect the brake operating speed based on output values obtained from the plurality of filters.
It is assumed herein that a certain filter has a cut-off frequency f. If the cut-off frequency f is higher than the rising speed of a vehicle body deceleration as the input value of the filter, the output value of the filter is not affected by the cut-off frequency f. In such a situation, the input value input to the filter is approximately equal to the output value output from the filter. On the other hand, if the cut-off frequency f of the filter is lower than the rising speed of the vehicle body deceleration, the output value of the filter is affected by the cut-off frequency f. In such a situation, the output value of the filter changes with delay in response to a change in the input value of the filter. As a result, the change in the output value is damped with respect to the change in the input value. Accordingly, for example, if vehicle body decelerations sequentially calculated based on the detected rotational speeds of the wheels are input to a plurality of filters having different cut-off frequencies, and if a filter whose output value tends to be damped is identified, the frequency higher than the cut-off frequency of the filter by a predetermined value corresponds to the rising speed indicative of an increasing state of the vehicle body deceleration. The rising speed of the vehicle body deceleration corresponds to the brake operating speed, namely, the speed at which the brake pedal is depressed. Thus, using such relations, the brake operating speed can be detected based on output values obtained through the respective filters.
Thus, it is possible to detect the brake operating speed based on the wheel speeds already detected during ABS control or the like, without employing a pressure sensor for directly detecting the brake fluid pressure.
In the brake operating speed detecting device according to the first aspect of the invention, the brake operating speed may be detected based on the state of distribution of output values obtained through the respective filters.
If vehicle body decelerations sequentially calculated based on the detected rotational speeds of the wheels are input to the respective filters, the output values of those filters whose cut-off frequency is higher than the rising speed of the vehicle body deceleration are hardly damped and remain within a predetermined range A. On the other hand, in those filters whose cut-off frequency is lower than the rising speed of the vehicle body deceleration, the input value is damped to a greater extent as the set cut-off frequency lowers. For this reason, the output value departs from the predetermined range A to a degree corresponding to the cut-off frequency and becomes discrete. Based on such a state of distribution of the output values of the filters, it is possible to specify a frequency corresponding to the rising speed of the vehicle body deceleration and to thereby detect the brake operating speed from the specified frequency.
In the brake operating speed detecting device according to the first aspect of the invention, the brake operating speed may be detected based on the difference between output values obtained through the respective filters.
The output value of a filter having a cut-off frequency higher than the rising speed of the vehicle body deceleration is different from the output value of a filter having a cutoff frequency lower than the rising speed of the vehicle body deceleration. Because this difference changes in accordance with the increasing state of the vehicle body deceleration, namely, the brake operating speed, it is possible to detect the brake operating speed based on the difference between the output values of the filters.
A brake control device according to a second aspect of the invention is designed to: (a) filter the vehicle body deceleration obtained based on the state of changes in the rotational speeds of wheels by means of a plurality of filters having different cut-off frequencies, (b) detect the brake operating speed based on output values obtained from the plurality of filters, and (c) start braking force distribution control if it is judged based on the detected brake operating speed that a hard braking operation has been performed.
The brake operating speed is detected based on the output values obtained through the respective filters. If it is judged based on the detected brake operating speed that a hard braking operation has been performed, braking force distribution control is started. Thereby operation control of an actuator is performed immediately such that the braking forces applied to the rear wheels are reduced with respect to the braking forces applied to the front wheels.
By this operation, it becomes possible to detect that a hard braking operation has been performed, without employing a pressure sensor for directly detecting the brake fluid pressure. If a hard braking operation has been performed, it becomes possible to immediately perform operation control of the actuator for reducing the braking forces applied to the rear wheels with respect to the braking forces applied to the front wheels. Therefore, adjustment of the braking forces applied to the rear wheels can be started at a suitable timing corresponding to the brake operating speed. Consequently, it becomes possible to prevent the rear wheels from being locked even if a hard braking operation has been performed.
A brake control device according to a third aspect of the invention is designed to: (a) filter a vehicle body deceleration obtained based on the state of change in the rotational speeds of wheels by means of a plurality of filters having different cut-off frequencies, and (b) detect the brake operating speed based on output values obtained from the plurality of filters. By comparing the rotational speeds of the front and rear wheels with each other, the state where the rotational speeds of the rear wheels have been reduced with respect to the rotational speeds of the front wheels by a predetermined value or more is detected. Braking force distribution control is started based on the state of the detected brake operating speed and rotational speeds.
In braking force distribution control, the braking forces generated in the rear wheels are reduced with respect to the braking forces generated in the front wheels by controlling fluid pressures for generating the braking forces in the respective wheels.
In the case where the timing for starting braking force distribution control is judged based merely on the rotational speeds of the wheels, braking force distribution control is not started until the rotational speeds of the rear wheels actually fall and the difference between the rotational speeds of the front and rear wheels becomes equal to or greater than a predetermined value. However, if a hard braking operation has been performed, the condition for starting ABS control may be fulfilled before the difference between the rotational speeds of the front and rear wheels becomes equal to or greater than the predetermined value. Thus, the timing for starting braking force distribution control is determined considering both the detection result of performance of a hard braking operation and the detection result of a fall in the rotational speeds of the rear wheels.
By this operation, it becomes possible to surely start control by a braking force distribution controller if a hard braking operation has been performed, even before the difference between the rotational speeds of the front and rear wheels becomes equal to or greater than a predetermined value.
The aspects of the invention are not limited to the brake operating speed detecting device and the brake control device as mentioned above. Further aspects of the invention include, for example, a vehicle equipped with a brake operating speed detecting device or a brake control device, and a brake operating speed detecting method or a brake control method.